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Lead-free carbon brushes for automotive starters
Wear 255 (2003) 1286–1290
Case study
Lead-free carbon brushes for automotive starters Arwed Uecker∗ Carbone Lorraine, Deutsche Carbone AG, Frankfurt, Germany
Abstract Historically, carbon brushes for automotive starters contain lead as an additive. The desired effect is an increase in starter performance without raising the copper to current-density ratio, which is directly correlated to the brush wear rate. However, lead dust is toxic or may cause genetic changes if incorporated, even in small quantities. In addition, lead oxidizes easily in air, which leads to dimensional and resistivity changes of the brushes over time. The role of lead in the contact has been attributed to its ability to control the film, and lowering the temperature by dissipating heat through phase transition. Only zinc and tin fulfill the requirements of non-toxicity, low melting point and low hardness of the oxide. Tests were performed, and it turned out to be necessary to separate the additive from the copper, to prevent complete migration into the copper and loss of its necessary properties as a metal or oxide. Several different methods were tested to make zinc a usable replacement for lead and the final version yields a lower wear rate with increased performance. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbon brush; Wear; Starter; Lead
1. Introduction Since its introduction almost 100 years ago, the electrical starter motor for automobiles went through numerous changes [1]. Most of them were driven by the desire to improve durability and the performance to weight ratio, however, others were initiated by changes in the environmental requirements. One of the recent environmental requirements, which will take effect in 2003 in Europe and will likely be followed with local regulations in the US, is the suppression of lead in all automotive components that could create airborne dust. In addition to its well known toxicity, lead has been found to be a source for genetic alterations in humans. The low voltage in automotive applications requires carbon brushes with very low contact resistance, which is particularly necessary for high power starter motors, which carry peak current densities of 1000 A/cm2 . In order to achieve the required low resistivity, metal filled carbon brushes are used. Depending on the supplied voltage and the type of starter, metal contents of 50–95% are used. The remainder consists of agglomerated graphite and other forms of solid lubricants and abrasives. Most of the metal is copper with the addition of lead; sometimes silver, iron, or manganese is added [2–5]. ∗ Present address: Carbone Lorraine, 300 West Industrial Park Road, Farmville, VA 23901, USA. Tel.: +1-434-395-8212; fax: +1-434-395-8285. E-mail address: [email protected] (A. Uecker).
The lubricating effect (e.g. for intake valves of gasoline engines) of lead is well known and came into public awareness with the ban of lead as an additive for gasoline in the early 1970s. At that time, engine manufacturers had to find new materials for intake valves. The carbon brush industry is facing a similar lubrication issue now with the ban of lead in their products. The addition of lead to carbon brushes is advantageous for several reasons. For brushes that are heat treated below the sintering temperature of copper, a significant amount of lead increases the strength of the material and lowers the resistivity. This is due to the fact that the lead solders the copper particles together. The mild abrasive action of the lead oxide stabilizes the contact drop in high current applications and lead allows overcharging the contact for short periods of time, which is an essential benefit for starter applications. However, the oxidation of lead within the brush material can sometimes lead to cracks and increased resistivity over time. The role of lead in sliding contacts has been attributed mainly to two physical properties: the low melting point and the softness of its oxide. The low melting point allows the lead within and around the contact area to undergo a heat dissipating phase transition. This in turn keeps the contact area cooler and more stable than without the lead and, consequently, allows higher current densities. The molten lead also prevents copper welding [6]; without the lead the copper particles from the brush weld to the copper commutator and are being pulled out of the brush by the sliding action,
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00182-0
A. Uecker / Wear 255 (2003) 1286–1290 Table 1 Additives to replace lead in carbon brushes Metal
Melting point (K)
Mohs hardness of metal
Mohs hardness of oxide
Copper Lead Aluminum Zinc Tin
1356 600 932 693 505
3 1.5 2.3 2.5 1.7
3.7 2 9 4.3 6.7
causing increased wear. The soft lead oxide on the other hand acts as a mild abrasive and helps forming and maintaining a stable, thin film. Materials which could possibly replace lead would have to be cheap, conductive, and have a low melting point, a soft oxide, and a low toxic potential (Table 1). Aluminum has a
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low melting point, but is ruled out because of a very hard oxide. The only true candidates are tin and zinc. Tin and zinc both have high diffusion rates in copper. The diffusion rate of tin, even at ambient temperature is so high that it is impossible to put tin and copper in mechanical contact without forming an alloy within a few days. Unfortunately, these alloys have a high melting point and cannot act as a heat sink. The situation is not as critical with zinc, however, the diffusion has to be inhibited during the heat treatment.
2. Sample preparation Brushes were manufactured following a standard industrial process (Fig. 1). In a first step, 92% of natural flake graphite powder is mixed with 8% phenolic resin in solution,
Fig. 1. Manufacturing process for automotive brushes.
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A. Uecker / Wear 255 (2003) 1286–1290
Fig. 2. Brush dimensions.
the mixture is then dried and milled. In a second step, this premix is blended with electrolytic copper, fine lead powder, and other necessary additives in a ratio of 28% premix, 62% copper, 4% lead and 5% MoS2 (all percentage by weight). In a third step, the final blend is molded to a near-shape brush at 0.3 GPa (the shunt wire is molded in), which is then, in a fourth step, sintered at 900 K. The sintering atmosphere is a mixture of 25% hydrogen and the balance nitrogen, to provide a reducing environment which aids the sintering process and reduces any copper oxide. Finally, the brush is machined to the desired shape: 18 mm wide (a), 7.4 mm long (t), 19 mm tall (r), with a useable length of 9 mm (Fig. 2).
3. Experimental setup For starter brushes, a very close relationship exists between performance and brush wear. It is, therefore, crucial
to always measure both parameters. As brush wear depends heavily on the environment provided by the motor, no standardized brush wear tester can adequately determine durability. Therefore, all tests were performed on one specific motor type, a Bosch 2.2 kW, gear reduced, wound field starter, which yields 2.74 ± 0.06 kW with the control sample and has a durability expectation of minimum 40,000 cycles. All performance tests were done on the same motor with a Vibrometer WB-PB115 powder brake performance tester from no-load to stall-torque. The maximum output power was calculated from speed and torque measurements. Three tests were performed with one set of brushes for each material and the average and standard deviation were computed. Durability tests were performed by mounting the starter with the test brushes on a mechanical simulator which mimics the load cycle of a gasoline engine during the start-up process (Fig. 3). The total cycle consisted of 1 s crank, 1 s overrun, and 28 s rest. The cycle was performed at least
Fig. 3. Engine simulator for wear testing.
A. Uecker / Wear 255 (2003) 1286–1290
1289
Table 2 Test results of different additives in starter brushes Number
Description
Performancea (kW)
735 01 735A68 735A69 735A70 735A91 735A92 835A12 735A87 635A07 635A08 635A09 635A10 735B11 735B12
4% Lead in final blend 4% Lead in premix 4% Tin in premix 4% Zinc in premix 0.2% Zinc in premix 1% Zinc in premix 8% Zinc in premix 4% Zinc in final blend 0.3% ZnCO3 in final blend 1% ZnCO3 in final blend 3% ZnCO3 in final blend 6% ZnCO3 in final blend 4% ZnO in premix 4% ZnO in final blend
2.78 2.82 2.86 2.85 2.85 2.84 2.83 2.81 2.83 2.70 2.88 2.88 2.86 2.85
a b
± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.04 0.08 0.10 0.03 0.02 0.03 0.04 0.08 0.04 0.09 0.03 0.10 0.02 0.02
Wear rate (m per cycle)
Durabilityb (cycles)
0.21
42000
0.19
48000
0.15
62000
x ± s from three tests. Extrapolated after 30,000 cycles.
Fig. 4. Wear track of starter brushes on commutator.
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A. Uecker / Wear 255 (2003) 1286–1290
30,000 times before the motor was disassembled and the brushes removed for evaluation. The absolute wear was measured with a micrometer for each brush and the projected brush life was calculated. As brush wear is fairly linear with time, durability can be extrapolated from the number of cycles tested, absolute wear of the brush that wore most, and the usable brush length. One test was performed for each composition. Both testers were equipped with a battery-simulating power supply. Tests were performed at room temperature.
4. Experimental results The easiest way of inhibiting alloy forming was to separate the copper from the additive by adding the zinc or tin into the premix. This traps most of the zinc or tin within the graphite/resin agglomerates. Copper is added to the final blend, which reduces the contact of copper and tin or zinc to a minimum. These materials were tested versus the standard grade with lead in the final blend. For comparison, a sample with lead in the premix was made which used the same process parameters as the tin and zinc grades. The results in terms of performance were encouraging, both new materials matched or exceeded the lead-containing grade (Grades 735 01, 735A68, 735A69 and 735A70; Table 2). At this point, we decided to continue our work with zinc rather than tin for the lesser danger of zinc to alloy with copper and the greater softness of its oxide. In order to improve the process, the amount of zinc in the black mix was varied, however, we did not find a significant variation in performance between 0.2 and 8% of zinc (Grades 735A70, 735A91, 735A92 and 835A12; Table 2). The data show that even a small amount of zinc is enough to give a reasonable performance; however, as it is easier to work with, we continued to use 4%. Life testing was performed on the 4% zinc grade which yielded 14% longer life than the control, despite its higher performance. In order to confirm that the alloy does not work as well as the metal itself, we added the zinc to the final blend, where it comes in intimate contact with the copper. The result was, as expected, a drop in output power compared to the grade with elemental zinc, which showed that the complete alloying which took place was detrimental to the performance (Grade 735A87; Table 2). In order to further understand the process and improve the contact, other different ways to separate zinc from copper were investigated. Since the baking takes place in a reducing atmosphere, zinc could be added by way of zinc oxide or zinc carbonate. Both methods worked well (Grades 635A07, 635A08, 635A09, 635A10, 735B11 and 735B12; Table 2). As ZnCO3 yielded the highest performance of all materials, it was tested in life. The results were 62,000 cycles, 48% longer than the standard grade with lead. In addition, the commutator wear was reduced (Fig. 4).
5. Discussion When zinc was found to be a good substitute for lead, the mechanism of improving performance and lifetime was not well understood: it could be the elemental zinc, the zinc–copper alloy, or the zinc oxide which boosts performance and durability. The experimental results showed that forced alloying is detrimental to the performance; supplemental tests with different sintering profiles supported these findings. However, if the zinc is introduced in a way, that allows only little contact and promotes elementary zinc in the vicinity of the copper, the positive effect was strongest. This could be achieved by mechanical separation with little diffusion during baking or via “retarded” release of the zinc, when reduced from an oxide, or in a two-step process from a carbonate, late in the sintering process. As zinc has a very high vapor pressure at temperatures above 823 K, which is far below the boiling point of 1180 K, migration effects cannot be suppressed during heat treatment completely. The solubility of zinc in copper, on the other hand, is so high, that the evaporated zinc will not leave the brush, but will form an alloy or just condense on the copper surface. The zinc–copper alloy has better stability against oxidation [7], and helps to prevent premature oxidation of the sliding surface at high current densities. This was confirmed through thermogravimetric analysis, which showed clearly that the oxidation of the zinc containing material started at 513 K, whereas the oxidation of pure copper material already started at 423 K.
Acknowledgements This work was carried out at the Carbone Lorraine laboratory at Deutsche Carbone AG in Frankfurt, Germany. The author thanks the staff of the motor test facility, as well as the analytical laboratory, for their support in this study. References [1] K. Kamai, Environmentally-friendly products by Denso, Denso Tech. Rev. 7 (1) (2002) 17. [2] T.W.S. Chow, H.W. Bishop, Wear 126 (1988) 1–15. [3] E. Strobel, E. Kühne, Erfahrungen mit Schleifkontakten aus Eisen-Blei-Graphit-Verbundwerkstoffen für Oberleitungsomnibusse, Kontaktwerkstoffe in der Elektrotechnik, Akademie Verlag, Berlin, 1962, pp. 230–234. [4] T. Ichiki, Theory and Practice of Carbon Brushes, Toyo Tanso, Tokyo, Japan, 1978, 6 pp. [5] Motor Brush and its Application, NDC541.623, Carbon Brush Institute, 5 April 1976, 16 pp. [6] H. Schreiner, Pulvermetallurgie elektr. Kontakte, Springer Verlag, Berlin, 1964, pp. 142–145. [7] R.F. Tylecote, Review of published information on the oxidation and scaling of copper and copper based alloys, J. Inst. Met. 78 (1950) 259–300.
Case study
Lead-free carbon brushes for automotive starters Arwed Uecker∗ Carbone Lorraine, Deutsche Carbone AG, Frankfurt, Germany
Abstract Historically, carbon brushes for automotive starters contain lead as an additive. The desired effect is an increase in starter performance without raising the copper to current-density ratio, which is directly correlated to the brush wear rate. However, lead dust is toxic or may cause genetic changes if incorporated, even in small quantities. In addition, lead oxidizes easily in air, which leads to dimensional and resistivity changes of the brushes over time. The role of lead in the contact has been attributed to its ability to control the film, and lowering the temperature by dissipating heat through phase transition. Only zinc and tin fulfill the requirements of non-toxicity, low melting point and low hardness of the oxide. Tests were performed, and it turned out to be necessary to separate the additive from the copper, to prevent complete migration into the copper and loss of its necessary properties as a metal or oxide. Several different methods were tested to make zinc a usable replacement for lead and the final version yields a lower wear rate with increased performance. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbon brush; Wear; Starter; Lead
1. Introduction Since its introduction almost 100 years ago, the electrical starter motor for automobiles went through numerous changes [1]. Most of them were driven by the desire to improve durability and the performance to weight ratio, however, others were initiated by changes in the environmental requirements. One of the recent environmental requirements, which will take effect in 2003 in Europe and will likely be followed with local regulations in the US, is the suppression of lead in all automotive components that could create airborne dust. In addition to its well known toxicity, lead has been found to be a source for genetic alterations in humans. The low voltage in automotive applications requires carbon brushes with very low contact resistance, which is particularly necessary for high power starter motors, which carry peak current densities of 1000 A/cm2 . In order to achieve the required low resistivity, metal filled carbon brushes are used. Depending on the supplied voltage and the type of starter, metal contents of 50–95% are used. The remainder consists of agglomerated graphite and other forms of solid lubricants and abrasives. Most of the metal is copper with the addition of lead; sometimes silver, iron, or manganese is added [2–5]. ∗ Present address: Carbone Lorraine, 300 West Industrial Park Road, Farmville, VA 23901, USA. Tel.: +1-434-395-8212; fax: +1-434-395-8285. E-mail address: [email protected] (A. Uecker).
The lubricating effect (e.g. for intake valves of gasoline engines) of lead is well known and came into public awareness with the ban of lead as an additive for gasoline in the early 1970s. At that time, engine manufacturers had to find new materials for intake valves. The carbon brush industry is facing a similar lubrication issue now with the ban of lead in their products. The addition of lead to carbon brushes is advantageous for several reasons. For brushes that are heat treated below the sintering temperature of copper, a significant amount of lead increases the strength of the material and lowers the resistivity. This is due to the fact that the lead solders the copper particles together. The mild abrasive action of the lead oxide stabilizes the contact drop in high current applications and lead allows overcharging the contact for short periods of time, which is an essential benefit for starter applications. However, the oxidation of lead within the brush material can sometimes lead to cracks and increased resistivity over time. The role of lead in sliding contacts has been attributed mainly to two physical properties: the low melting point and the softness of its oxide. The low melting point allows the lead within and around the contact area to undergo a heat dissipating phase transition. This in turn keeps the contact area cooler and more stable than without the lead and, consequently, allows higher current densities. The molten lead also prevents copper welding [6]; without the lead the copper particles from the brush weld to the copper commutator and are being pulled out of the brush by the sliding action,
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00182-0
A. Uecker / Wear 255 (2003) 1286–1290 Table 1 Additives to replace lead in carbon brushes Metal
Melting point (K)
Mohs hardness of metal
Mohs hardness of oxide
Copper Lead Aluminum Zinc Tin
1356 600 932 693 505
3 1.5 2.3 2.5 1.7
3.7 2 9 4.3 6.7
causing increased wear. The soft lead oxide on the other hand acts as a mild abrasive and helps forming and maintaining a stable, thin film. Materials which could possibly replace lead would have to be cheap, conductive, and have a low melting point, a soft oxide, and a low toxic potential (Table 1). Aluminum has a
1287
low melting point, but is ruled out because of a very hard oxide. The only true candidates are tin and zinc. Tin and zinc both have high diffusion rates in copper. The diffusion rate of tin, even at ambient temperature is so high that it is impossible to put tin and copper in mechanical contact without forming an alloy within a few days. Unfortunately, these alloys have a high melting point and cannot act as a heat sink. The situation is not as critical with zinc, however, the diffusion has to be inhibited during the heat treatment.
2. Sample preparation Brushes were manufactured following a standard industrial process (Fig. 1). In a first step, 92% of natural flake graphite powder is mixed with 8% phenolic resin in solution,
Fig. 1. Manufacturing process for automotive brushes.
1288
A. Uecker / Wear 255 (2003) 1286–1290
Fig. 2. Brush dimensions.
the mixture is then dried and milled. In a second step, this premix is blended with electrolytic copper, fine lead powder, and other necessary additives in a ratio of 28% premix, 62% copper, 4% lead and 5% MoS2 (all percentage by weight). In a third step, the final blend is molded to a near-shape brush at 0.3 GPa (the shunt wire is molded in), which is then, in a fourth step, sintered at 900 K. The sintering atmosphere is a mixture of 25% hydrogen and the balance nitrogen, to provide a reducing environment which aids the sintering process and reduces any copper oxide. Finally, the brush is machined to the desired shape: 18 mm wide (a), 7.4 mm long (t), 19 mm tall (r), with a useable length of 9 mm (Fig. 2).
3. Experimental setup For starter brushes, a very close relationship exists between performance and brush wear. It is, therefore, crucial
to always measure both parameters. As brush wear depends heavily on the environment provided by the motor, no standardized brush wear tester can adequately determine durability. Therefore, all tests were performed on one specific motor type, a Bosch 2.2 kW, gear reduced, wound field starter, which yields 2.74 ± 0.06 kW with the control sample and has a durability expectation of minimum 40,000 cycles. All performance tests were done on the same motor with a Vibrometer WB-PB115 powder brake performance tester from no-load to stall-torque. The maximum output power was calculated from speed and torque measurements. Three tests were performed with one set of brushes for each material and the average and standard deviation were computed. Durability tests were performed by mounting the starter with the test brushes on a mechanical simulator which mimics the load cycle of a gasoline engine during the start-up process (Fig. 3). The total cycle consisted of 1 s crank, 1 s overrun, and 28 s rest. The cycle was performed at least
Fig. 3. Engine simulator for wear testing.
A. Uecker / Wear 255 (2003) 1286–1290
1289
Table 2 Test results of different additives in starter brushes Number
Description
Performancea (kW)
735 01 735A68 735A69 735A70 735A91 735A92 835A12 735A87 635A07 635A08 635A09 635A10 735B11 735B12
4% Lead in final blend 4% Lead in premix 4% Tin in premix 4% Zinc in premix 0.2% Zinc in premix 1% Zinc in premix 8% Zinc in premix 4% Zinc in final blend 0.3% ZnCO3 in final blend 1% ZnCO3 in final blend 3% ZnCO3 in final blend 6% ZnCO3 in final blend 4% ZnO in premix 4% ZnO in final blend
2.78 2.82 2.86 2.85 2.85 2.84 2.83 2.81 2.83 2.70 2.88 2.88 2.86 2.85
a b
± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.04 0.08 0.10 0.03 0.02 0.03 0.04 0.08 0.04 0.09 0.03 0.10 0.02 0.02
Wear rate (m per cycle)
Durabilityb (cycles)
0.21
42000
0.19
48000
0.15
62000
x ± s from three tests. Extrapolated after 30,000 cycles.
Fig. 4. Wear track of starter brushes on commutator.
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A. Uecker / Wear 255 (2003) 1286–1290
30,000 times before the motor was disassembled and the brushes removed for evaluation. The absolute wear was measured with a micrometer for each brush and the projected brush life was calculated. As brush wear is fairly linear with time, durability can be extrapolated from the number of cycles tested, absolute wear of the brush that wore most, and the usable brush length. One test was performed for each composition. Both testers were equipped with a battery-simulating power supply. Tests were performed at room temperature.
4. Experimental results The easiest way of inhibiting alloy forming was to separate the copper from the additive by adding the zinc or tin into the premix. This traps most of the zinc or tin within the graphite/resin agglomerates. Copper is added to the final blend, which reduces the contact of copper and tin or zinc to a minimum. These materials were tested versus the standard grade with lead in the final blend. For comparison, a sample with lead in the premix was made which used the same process parameters as the tin and zinc grades. The results in terms of performance were encouraging, both new materials matched or exceeded the lead-containing grade (Grades 735 01, 735A68, 735A69 and 735A70; Table 2). At this point, we decided to continue our work with zinc rather than tin for the lesser danger of zinc to alloy with copper and the greater softness of its oxide. In order to improve the process, the amount of zinc in the black mix was varied, however, we did not find a significant variation in performance between 0.2 and 8% of zinc (Grades 735A70, 735A91, 735A92 and 835A12; Table 2). The data show that even a small amount of zinc is enough to give a reasonable performance; however, as it is easier to work with, we continued to use 4%. Life testing was performed on the 4% zinc grade which yielded 14% longer life than the control, despite its higher performance. In order to confirm that the alloy does not work as well as the metal itself, we added the zinc to the final blend, where it comes in intimate contact with the copper. The result was, as expected, a drop in output power compared to the grade with elemental zinc, which showed that the complete alloying which took place was detrimental to the performance (Grade 735A87; Table 2). In order to further understand the process and improve the contact, other different ways to separate zinc from copper were investigated. Since the baking takes place in a reducing atmosphere, zinc could be added by way of zinc oxide or zinc carbonate. Both methods worked well (Grades 635A07, 635A08, 635A09, 635A10, 735B11 and 735B12; Table 2). As ZnCO3 yielded the highest performance of all materials, it was tested in life. The results were 62,000 cycles, 48% longer than the standard grade with lead. In addition, the commutator wear was reduced (Fig. 4).
5. Discussion When zinc was found to be a good substitute for lead, the mechanism of improving performance and lifetime was not well understood: it could be the elemental zinc, the zinc–copper alloy, or the zinc oxide which boosts performance and durability. The experimental results showed that forced alloying is detrimental to the performance; supplemental tests with different sintering profiles supported these findings. However, if the zinc is introduced in a way, that allows only little contact and promotes elementary zinc in the vicinity of the copper, the positive effect was strongest. This could be achieved by mechanical separation with little diffusion during baking or via “retarded” release of the zinc, when reduced from an oxide, or in a two-step process from a carbonate, late in the sintering process. As zinc has a very high vapor pressure at temperatures above 823 K, which is far below the boiling point of 1180 K, migration effects cannot be suppressed during heat treatment completely. The solubility of zinc in copper, on the other hand, is so high, that the evaporated zinc will not leave the brush, but will form an alloy or just condense on the copper surface. The zinc–copper alloy has better stability against oxidation [7], and helps to prevent premature oxidation of the sliding surface at high current densities. This was confirmed through thermogravimetric analysis, which showed clearly that the oxidation of the zinc containing material started at 513 K, whereas the oxidation of pure copper material already started at 423 K.
Acknowledgements This work was carried out at the Carbone Lorraine laboratory at Deutsche Carbone AG in Frankfurt, Germany. The author thanks the staff of the motor test facility, as well as the analytical laboratory, for their support in this study. References [1] K. Kamai, Environmentally-friendly products by Denso, Denso Tech. Rev. 7 (1) (2002) 17. [2] T.W.S. Chow, H.W. Bishop, Wear 126 (1988) 1–15. [3] E. Strobel, E. Kühne, Erfahrungen mit Schleifkontakten aus Eisen-Blei-Graphit-Verbundwerkstoffen für Oberleitungsomnibusse, Kontaktwerkstoffe in der Elektrotechnik, Akademie Verlag, Berlin, 1962, pp. 230–234. [4] T. Ichiki, Theory and Practice of Carbon Brushes, Toyo Tanso, Tokyo, Japan, 1978, 6 pp. [5] Motor Brush and its Application, NDC541.623, Carbon Brush Institute, 5 April 1976, 16 pp. [6] H. Schreiner, Pulvermetallurgie elektr. Kontakte, Springer Verlag, Berlin, 1964, pp. 142–145. [7] R.F. Tylecote, Review of published information on the oxidation and scaling of copper and copper based alloys, J. Inst. Met. 78 (1950) 259–300.